Combustion control system



Feb. 20, 1968 G. M.IBAUDELET DE LIVOIS 3,369,748

COMBUSTION CONTROL SYSTEM Filed March 28, 1966 mdE INVENTOR. GUY M. BAUDELET de LIVOIS United States Patent 3,369,748 COMBUSTION CONTROL SYSTEM Guy M. Baudelet de Livois, Paris, France, assignor to Controle Bailey (Societe Anonyme), a company of France Filed Mar. 28, 1966, Ser. No. 537,786 Claims priority, application France, Aug. 14, 1965,

28,388 1 Claim. (Cl. 236-15) ABSTRACT OF THE DISCLOSURE A combustion control system in which air flow is in ratio to fuel flow but readjusted from the larger of two signals, one of which is proportional to the oxygen content in the products of combustion and the other of which is proportional to the opacity of the products of combustion.

The present invention relates to a regulating chain for the combustion of fuel oil with a very slight excess of air, or more precisely to a chain for regulating the excess of air used for the combustion of a heating fuel-oil with a view to maintaining this excess of air at a strict minimum.

The relatively high sulphur content of heavy industrial heating fuel oils is the source of difliculties resulting essentially from the corrosion suffered by the walls of heat exchangers because when these are sufliciently cold for the water vapor of combustion to condense thereon, the water thus formed dissolves the sulphuric anhydride S0 present, producing incipient sulphuric acid.

These difliculties are particularly serious in boilers where the high yield demanded necessitates extensive cooling of the flue gases evolved.

In order to solve the problem thus posed, it has already been suggested that the excess of air admitted to the furnace should be reduced to a strict minimum in such a manner as to prevent the presence of S0 in the flue gases given off, at least to a great extent and substantially completely. Experience has, in fact, shown that when the excess of combustion air is less than 3 or 4%, which corresponds to a residual oxygen content of 0.6 to 0.8%, there is substantially no further formation of sulphuric acid and, in addition, a very considerable lowering in the dew point is observed. These two results contribute to reducing considerably the corrosion of the cold surfaces of the heat exchangers.

Unfortunately, it is really diflicult to maintain a very slight excess of air constant, particularly when the boiler load is liable to rapid and great variations, as it is frequently the case.

In fact, only automatic regulation can provide an appropriate solution, provided that it can be guided by devices for detecting an excess of air which are sufliciently sensitive, precise and reliable.

In particular, the conventional method of regulation which consists in regulating the measured supply of air and fuel oil in a suitable proportion at each moment has proved insufiiciently' precise and necessitates the cooperation of another source of data.

A first well-known solution consists in measuring the content of residual oxygen in the flue gases and in correcting the ratio of the supplies of air and of fuel oil in the appropriate sense as a function of said content of residual oxygen. This first solution may still prove inadequate, however, because three kinds of risk may arise. In the first place, the devices for measuring the oxygen content may deviate slightly and, as it has already been seen, it is suflicient for this content to vary by a few thousandths for a departure from the prescribed zone in one direction or the other, either with too great an excess 3,369,748 Patented Feb. 20, 1968 of air leading to the recurrence of the corrosion which it was desired to avoid, or with too slight an excess of air causing the appearance of unburnt gases or solids residues. Secondly, the very precise oxygen analyzers which are necessary generally have response time constants which are ill-adapted to guide the regulation in a suitable manner in cases where the variations in load are rapid.

Finally, it is sometimes difficult to take a sample for analysis from the flue gases liberated which is truly representative of their average composition, particularly in the case of the admission of parasitic air.

Now there is a very sensitive, very precise and very reliable method of detecting the slightest lack of combustion air in oil-firing, which is simply to measure the opacity of the flue gases which are blackened by the unburnt hydrocarbons.

This mode of procedure has the following three advantages. In the first place, the measurement is very sensitive and a considerable deviation in the measuring instrument or opacimeter only corresponds to a very slight variation in the excess of air. On the other hand, the response time of such an instrument is much shorter than that of an oxygen analyzer. Finally, the average measuremeasurement of the opacity of the flue gases releasedavoids the three risks indicated above, involved in that of the oxygen content of said flue gases.

The present invention relates essentially to a system of regulating the excess of air used in the combustion of a heating fuel-oil with a view to maintaining this excess of air at the strict minimum absolutely necessary.

For this purpose, according to the inveintion, the chain for regulating the excess of air is such that at least one of the regulating signals consists of a measurement signal for the opacity of the flue gases liberated, which signal is supplied by an opacimeter.

The invention will be better understood on reading the following description of three embodiments, given by way of example, with reference to the accompanying drawings in which:

FIG. 1 is a diagram of an oil-fired boiler;

FIG. 2 is a diagram of a regulating chain comprising, as its main regulating signal, that which is constituted by a flue-gas opacity measurement signal;

FIG. 3 is a diagram of a regulating chain comprising direct correction of the normal regulating signal by means of a flue-gas opacity measurement signal;

FIG. 4 is a regulating diagram comprising correction of the normal regulating signal by means of a measurement signal for the residual oxyen content in the flue gasesi supplemented by a flue-gas opacity measurement signa In FIG. 1, an oil-fired boiler comprises essentially an oil supply pipe 1, possibly with an oil fiowmeter 1a, an air supply pipe 2 possibly with an air tlowmeter 2a, said air being supplied by a blower 2b under the control of a servo-motor 2c regulating the blown air, then, downstream of the furnace 3, a steam outlet 4 and a flue-gas outlet 5, possibly with an oxygen analyzer 5x and, in any case, an opacimeter 5f. 7

In FIG. 2, the regulating chain only comprises the opacimeter 5] which controls the servo-motor 2c regulating the blown air by means of a regulating relay 6 with 3 In relatively large installations, it is preferable to use a more elaborate regulating chain in which the opacimeter only intervenes to correct a regulation efiected by other members.

In FIGS. 3 and 4, the regulating chain comprises essentially the oil flowrneter 10: associated with the air flowmeter 2a. The variations in flow of the two tfluids with respect to a predetermined ratio are found in a comparison relay which is an algebraic addition relay 2. This comparison relay transmits a regulating signal, which may be pneumatic or electric for example, to the servomotor 20 for regulating the blown air, by means of the relay 6 with proportional and integral action PI.

At the level of the comparison relay 2, this regulating chain in FIGS. 3 and 4 receives a correction signal which takes into consideration a flue-gas opacity measurement signal, produced by the opacimeter 5 In this manner, as soon as the opacity of the flue gases, which is slight but easily detected, deviates however slightly'from the average value selected as the index value, the conditions of comparison between the oil and air supplies are revised in the appropriate sense to achieve the final result aimed at.

In the embodiment in FIG. 3, the opacimeter 5f transmits this correction signal to the comparison relay by means of a relay 7 with a proportional and integral action PI.

In the embodiment in FIG. 4, the main correction signal reaching the comparison relay 2 does not originate from the opacimeter but from an oxygen analyzer 5x. This latter normally transmits a correction signal to a selector or comparator relay S which also receives information from the opacimeter 5 This selector or comparator relay S only retains the greater of these two input signals in order to transmit it to the adding relay 2.

The operation of this regulating chain is as follows. Normally, it is the oxygen analyzer 5x which ensures the final regulation of the combustion air. But if this correction signal proves too weak, either because of a deviation in the analyzer 5x or because of too long a response time therein, or for any reason, the opacimeter 5 provides a sort of relief for the oxygen analyzer so as to impose tem- 4 I v porarily, the appropriate adjustment of the excess combustion air. In this last embodiment, the opacimeter represents a sort of supplementary correcting member adapted to compensate for any failure in the main correcting member.

In the diagrams in FIGS. 3 and 4, the correction action of the opacimeter and of the oxygen analyzer is represented in the form of an algebraic addition applied to the comparison relay 2; but it could advantageously assume the form of a multiplier relay connected into the signal originating either from the oil flowmeter or the air flowmeter.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Means for regulating the supply mixture of oil and air of an oil-fired boiler comprising means for supplying air and means for supplying oil to said boiler, means for measuring the flow of air and means for measuring the flow of oil, means for varying at least the flow of said air supplying means, an oxygen analyzer in the flue-gas outlet of said boiler giving a first signal function of the oxygen content of said flue-gas, an opacimeter in the fine-gas outlet of said boiler giving a second signal function of the opacity of said flue-gas, comparator means for selecting the larger of said first and second signals, adding means for generating a third signal function of the measured air flow and oil flow and of said selected larger signal and a proportional and integral action relay operated by said third signal and controlling said varying means.

References Cited UNITED STATES PATENTS 2,285,564 6/1942 Brooke et a1. 236-15 2,407,838 9/ 1946 Kliever 236-15 2,441,025 5/1948 Logan 23615 X 3,184,686 5/1965 Stanton 23615 X 3,216,661 11/1965 Sawyer 2361S ROBERT A. OLEARY, Primary Examiner.

W. E. WAYNER, Assistant Examiner. 

